The present invention relates to an optical apparatus including an aperture stop mechanism and an ND (neutral density) filter such as a video camera and a digital camera.
A light amount adjustment device included in an optical apparatus changes the area of a stop aperture using a plurality of stop blades to adjust the amount of light reaching an image-pickup element and obtain a proper exposure amount.
In order to prevent degradation of optical performance caused by so-called small aperture diffraction, the light amount adjustment device prevents the area of the stop aperture from being smaller than a certain small aperture, and sets a proper light amount in image pickup of a high brightness object using an ND filter covering the small aperture (see Japanese Patent Laid-Open No. 2002-55374).
However, in the conventional light amount adjustment device including the ND filter, the position of the ND filter is sometimes controlled so that the ND filter enters a state of covering only part of the small aperture, a so-called semi-covering state.
In such a semi-covering state, an opening (an uncovered opening) smaller than the small aperture is formed that is surrounded by a tip of the ND filter and an edge of the small aperture, and this small opening may cause small aperture diffraction. Thus, the ND filter cannot sufficiently prevent degradation of image quality caused by the small aperture diffraction.
Also, the ND filter is produced by vapor-depositing a film. In the semi-covering state, a step (a difference in optical path length) by a thickness of the board occurs on a transmission wavefront, thereby causing degradation of image quality.
Further, in order to reduce the size of the optical apparatus, a horizontal linear tip of the ND filter may be held in a position entering a full stop aperture. In this case, the tip of the ND filter exists in the stop aperture until the full stop aperture is narrowed down to some extent.
When a conventional CCD sensor is used as an image-pickup element and for example, a high brightness object in the form of a point light source exists in an image-pickup region, vertical smear occurs and the beam of light is generated by diffraction in a direction of an inclination angle of a linear portion of a stop blade that forms a stop aperture. Such smear and the beam of light can be prevented by using a smearless sensor such as a CMOS sensor as an image-pickup element.
However, as described above, when the horizontal linear tip of the ND filter enters the stop aperture, diffraction at the tip generates the beam of light in a vertical direction and degrades image quality even using a smearless image-pickup element.